In general, in an image forming apparatus of an electrophotographic system, plural photoconductive drums are arranged in parallel and laser beams are irradiated on the respective photoconductive drums to form electrostatic latent images. Toner images of respective colors are formed on the photoconductive drums by plural developing devices. The toner images of the respective colors are multiply transferred on to sheet paper to obtain a color image.
The developing devices are provided in the plural photoconductive drums. Plural toner cartridges are arranged to feed toners to the developing devices. The toners stored in the toner cartridges are carried to the developing devices.
In some image forming apparatus, sub-tanks are arranged between the toner cartridges and the developing devices. The sub-tanks are called sub-hoppers and are auxiliary toner tanks provided between the toner cartridges and the developing devices. Since the toners in the toner cartridges are fed to the sub-hoppers, even if there is no toner in the toner cartridges, it is possible to perform printing using the toners in the sub-hoppers.
JP-A-11-24513 discloses a toner supplying apparatus. In the toner supplying apparatus disclosed in JP-A-11-24513, sub-hoppers are arranged between toner cartridges and developing devices.
JP-A-2002-162809 discloses a color image forming apparatus. In the color image forming apparatus disclosed in JP-A-2002-162809, a capacity of a toner hopper for black having different frequency of use is set larger than a capacity of toner hoppers for the other three colors and a pitch among the toner hoppers for the three colors is minimized.
In the conventional image forming apparatus, the plural sub-hoppers are arranged at an interval same as an arrangement pitch of the developing devices. On the other hand, if the plural toner cartridges are arranged at the same interval and the size of the toner cartridge having high frequency of use is increased, the image forming apparatus is structurally increased in size.
If the interval among the plural toner cartridges is set smaller than the interval among the sub-hoppers, the image forming apparatus can be structurally reduced in size. However, since the interval among the toner cartridges and the interval among the sub-hoppers are different, paths for feeding the toners from the toner cartridges to the sub-hoppers are complicated and the number of components increases. When the sub-hoppers are replaced, since there are the plural sub-hoppers, the respective sub-hoppers may be attached in wrong positions.